The primary technology for capturing cells is magnetic beads. In this technology, a suspension of beads is used to treat a sample containing cells. The magnetic beads contain a tag or chemical entity that is selective for cells or for a certain cell type within the sample. After the cells become associated with the magnetic beads, a magnet is used to collect the magnetic beads and captured cells. The magnetic beads may be re-suspended several times with wash solutions to clean the cells. Finally, a solution may be used to release the cells from the beads. A magnetic is used to separate the magnetic beads from the cells.
The magnetic-activated cell sorting (MACS) method available from Miltenyl Biotec allows cells to be separated by incubating with magnetic nanoparticles coated with antibodies against a particular surface antigen. Cells expressing this surface antigen attach to the magnetic nanoparticles. Afterwards the cell solution is transferred on a column placed in a strong magnetic field. In this step, the cells attached to the nanoparticles (expressing the antigen) stay on the column, while other cells (not expressing the antigen) flow through.
With the MACS method, the cells can be separated either positively or negatively with respect to particular antigens. With positive selection, cells expressing the antigen(s) of interest, which attached to the magnetic column, are washed out to a separate vessel, after removing the column from the magnetic field. This method is useful for isolation of a particular cell type, for instance CD4 lymphocytes. In negative selection, the antibody used is directed against surface antigen(s) known to be present on cells that are not of interest. After administration of the cells/magnetic nanoparticles solution onto the column the cells expressing these antigens bind to the column and fraction that goes through is collected, as it contains almost no cells with undesired antigens.
Rare circulating tumor cells (CTCs) present in the bloodstream of patients with cancer provide a potentially accessible source for detection, characterization, and monitoring of nonhematological cancers. A microfluidic device, the Harvard CTC-Chip, has been used to capturing these epithelial cell adhesion molecule (EpCAM)-expressing cells using antibody-coated microposts. In a first generation device called 78,000 antibody-functionalized microposts were used to separate cells. Another microfluidic mixing device called the herringbone-chip is made up of parallel slanted channels (Li et al, Lab Chip, 13, 602).
In one technology, cells are collected on a flow through column at very slow flow rate and very low volumes making them difficult to use. The chip column does not contain a frit because the frit would prevent passage of the cells or trap, damage or kills the cells. But instead of a column frit a thin passage channel was used at the base of the chip to let the liquid flow through the column (Kralj et al. Lab Chip, 2012, 12, 4972-4975).
The EpCAM-based technique is very low throughput. A suspension of whole blood was pumped from 3 mL syringes at 0.2 mL/h for 1 h through the microfluidic packed bed to allow immobilization of the cancer cells. At this rate 5 hours is need to process 1 mL of blood. This may be an improvement over the Harvard CTC-chip, but still does not solve some fundamental issues with EpCAM based capture. The throughput is impractical and it too slow. Multiple channels can be used, but that will dramatically increase the imaging and staining area, making it difficult to perform high resolution cytomorphological analysis
These magnetic bead methods and micro or chip based columns are slow and do not produce pure cell populations. There exists a need for a column technology that captures cells at high concentrations and then recovers the cells at high purity.